Features from optional dependencies

tokio implicit feature

Enables tokio

tokio:

An event-driven, non-blocking I/O platform for writing asynchronous I/O backed applications

Affects same-content::same_content_from_files_async, same-content::same_content_from_files_async2, same-content::same_content_from_readers_async, same-content::same_content_from_readers_async2